Isolation barriers are used in many industrial, medical and communication applications where it is necessary to electrically isolate two sections of electronic circuitry from one another. An electrical isolation barrier must exist, for example, in communication circuitry that is connected directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. In general, two sections of electronic circuitry are considered to be electrically isolated if a source of a large magnitude voltage (e.g., on the order of one thousand volts, or more), which is coupled on one side of the barrier, causes less than a minimal current flow (e.g., on the order of ten milliamperes, or less) through the barrier. High voltage isolation barriers commonly are implemented using transformers, capacitors, or opto-isolators, which transfer signals across the isolation barrier using magnetic fields, electric fields, or light, respectively.
In many applications, there exists an analog or continuous time varying signal on one side of the isolation barrier, and the information contained in that signal, as well as control information and synchronizing clock information, must be communicated across the isolation barrier. For example, common telephone network modulator/demodulator (or modem) circuitry, which is powered by a residential wall outlet, typically must transfer an analog signal with a bandwidth of approximately 4 kilohertz across an isolation barrier for transmission over the two-wire, public switched telephone network. In general, the isolation method and associated isolation circuitry should provide a reliable communication channel for the information to be conveyed across the isolation barrier. Thus, the isolating elements themselves should not significantly distort the signal information, the communication should be substantially insensitive to or undisturbed by voltage signals and impedances that exist between the isolated circuitry sections, and the communication should be substantially insensitive to or undisturbed by noise sources in physical proximity to the isolating elements.
Many different schemes for communicating analog signals across an isolation barrier have been proposed. Most of these approaches involve converting the analog signals into a digital format using pulse code modulation (PCM) techniques. For example, U.S. Pat. Nos. 5,500,894 and 5,602,912 describe a communication scheme in which an analog signal with information to be communicated across an isolation barrier is converted to a digital format, with the amplitude of the digital signal restricted to standard digital logic levels. The digital signal is transmitted across the barrier by means of two, separate high voltage isolation capacitors. One capacitor is used to transfer the digital signal logic levels, while a separate capacitor is used to transmit a clock or timing synchronization signal across the barrier. The clock signal is used on the receiving side of the barrier as a time base for analog signal recovery.
U.S. Pat. No. 4,901,275 describes a communication scheme in which an analog-to-digital converter (ADC) converts several, multiplexed analog channels into digital format for transmission to a digital system. Opto-isolators are used to isolate the ADC from electrical noise that is generated in the digital system. Serial data transmission across the isolation barrier is synchronized by a clock signal that is passed through a separate opto-isolator. The ADC clock is required for reliable signal reconstruction across the isolation barrier.
U.S. Pat. No. 6,225,927 describes an analog isolation system with digital communication across a capacitive barrier. In this approach, clock recovery circuitry may be employed on one side of the isolation barrier to extract timing information from the digital signal that is communicated across the barrier, and to filter the effects of phase noise that is introduced at the barrier. Delta-sigma converters may be disposed on both sides of the isolation barrier to convert signals between analog and digital domains. Bi-directional communication of digital signals is accomplished using a single pair of isolation capacitors. In preferred embodiments, the digital data communicated across the barrier consists of digital delta-sigma data signals multiplexed in time with other digital control information, signaling information, and framing information.